Horn antenna

ABSTRACT

A horn antenna for transmitting microwaves. A conical horn includes a small opening and a large opening. A transition piece includes a tapering portion and a non-tapering portion. The transition piece is arranged in the conical horn between a waveguide and an environment that the horn antenna is arranged in. A transition between the tapering portion and the non-tapering portion is arranged between the small opening and the large opening of the conical horn. The non-tapering portion engages the small opening of the conical horn.

FIELD OF THE INVENTION

The invention concerns a horn antenna for the directional transmissionof microwaves, where the antenna is constructed in such a way that itsoperation is not affected by condensation, derived from a fluid, whichhas accumulated on the antenna.

BACKGROUND OF THE INVENTION

Horn antennas for the transmission of microwaves are used in technology,for example, to measure the level of a fluid in a tank. In the case ofthis example such a device is normally positioned close to the roof ofthe tank. A wave guide for the transmission of microwaves to and fromthe antenna is connected to the antenna via an opening in the roof ofthe tank. In order to achieve a sealed boundary between the atmosphereprevailing in the chamber where the antenna is placed and the wave guideof the antenna, the antenna is generally equipped with a sealingtransitional piece between the wave guide and the antenna horn. So as toproduce a gentle transition this transition, piece is normally made ofteflon and conical in shape. However, the transition piece can haveother shapes, for example, a spherical knob, a plane surface or even acone-like part constructed by a stack-like arrangement of cylindricalbodies with progressively diminishing diameter. On account of itschemical stability, teflon is the preferred material for this part,although other materials may be used.

When deciding on the shape of the transition piece it became apparentthat this property has an influence on the antenna's ability to tolerateaccumulation of fluid on the surface of the antenna by, for example,condensation. Particularly vulnerable is the circular trench that existsbetween the transition piece, for example the cone, and the horn. Fluidcan easily accumulate here as a result of capillary action. Sincecertain fluids, for example water, have a high dielectric constant, anaccumulation of fluid around, for example, a conical transition piececould be interpreted, by the microwaves transmitted by the antenna, as arestriction. This is because the diameter of the wave guide is perceivedas reduced if fluid has ascended on to the envelope surface of the cone.

In this way the ability of the microwaves to propagate normally betweenthe wave guide and the environment is impeded.

Similarly, problems caused by condensation on the transition piece occurif this part has a different shape. For instance, if the transitionpiece is planar the accumulated moisture on this surface produces a moreor less indefinable wave guide diameter at this plane. Moreover, if theshape of the transition piece was such that it tapered in a stepwisefashion towards the antenna chamber then, even in this case the problemof moisture accumulations would arise in each individual step, which inturn would tend to produce restriction of the microwave. Owing to thesedifficulties, a conical transition piece is the most commonly occurringdesign within this technical field.

A transition piece, in the shape of a cone, from the wave guide to thechamber where the antenna is located is, according to current modelswithin microwave technology, designed so that the base of the cone is atapproximately the same level as the level where the cross-sectional areaof the horn antenna's cone begins to decrease, i.e. at the horn'snarrowest part. Following from this commonly adopted design, it has beenfound that condensed fluid accumulates in the aforementioned trench,between the horn and the transition piece, in an antenna that is used,for example, in the way mentioned above, i.e. in a tank in which a fluidis contained. The atmosphere in the tank is saturated, for exampleduring boiling. Furthermore, the temperature of the antenna is lowerthan that of its environment The effect of such circumstances is thatfluid in the tank condenses and falls on the antenna, causing therestriction in the trench that is present in the antenna, the result ofwhich is that the transmission of the microwaves is disturbed. Generallyspeaking, condensed moisture invariably appears on antennas of the typedescribed here in vapor-saturated tank environments; hence the problemof disturbed transmission is a recurring phenomena.

The invention presented here provides a solution to the problem outlinedabove.

SUMMARY OF THE INVENTION

According to one aspect of the invention a horn antenna for transmissionof microwaves is presented. The horn antenna includes a conical horn fordirectional transmission and reception of microwaves. At the center ofnarrowest part of the horn, a transition piece separating a wave guidefrom the environment is positioned. The transition piece is tapered andis characterized by its tapering part being arranged so as to operateoutside a normally fluid-filled circular trench between the horn and thetransition piece.

The transition piece is preferably designed as a radially symmetricalbody, of which its inner part is cylindrical, whereas its outer part isconical or cone-like, and thus tapering in the direction of theenvironment.

The cylindrical part of the transition piece constitutes a part extendedinto the horn. Between the cylindrical part of the transition piece andthe horn is the previously mentioned trench, where fluid accumulates asa result of condensation. In the case of a downward facing antenna withthe horn and the transition piece directed downwards, it is thuspossible to predict the length of the cylindrical part so that the fluiddraining from the trench is unable to be retained by capillary action.In this way the length of the cylindrical part can be determined so thatthe remaining fluid in the trench does not appear further out on thetransition piece, where it tapers off.

An advantage of such a design for the transition piece is that the fluidpresent in the trench exists only along the transition piece's uniformthickness. Owing to the fluid's normally relatively high dielectricconstant, the accumulated fluid is interpreted by the microwavestransmitted through the transition piece as part of the wave guide.Consequently, the wave guide diameter is not affected by the fluidaccumulation, which in turn means that use of the horn antenna isindependent of the presence of fluid in the trench. The design of thetransition piece does not prevent the accumulation of fluid in thespace, i.e. the trench between horn and transition piece. Nevertheless,consideration has been taken of the fact that fluid is normally presentduring employment of the antenna and that the antenna is designedaccordingly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventionally designed horn antenna

FIG. 2 illustrates a horn antenna designed to incorporate the inventiondescribed here.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described below with the aid of the attached figures.

In FIG. 1 a horn antenna 1 of conventional type, used for example togauge the level of a fluid in tanks, is shown. The antenna here isassembled according to the figure. i.e. with the conical horn 2 facingdownwards. A wave guide, omitted from the figure, is attached to thehorn antenna 1 with a transition piece 3 arranged at the connection ofthe wave guide to the antenna. The transition piece 3 is inserted in andseals an opening at the narrowest part of the horn 2. The conventionallyarranged transition piece 3 is designed so that it tapers towards theouter part of the horn 2, usually in the shape of a circular cone. Thetransition piece 3 is, as mentioned previously, preferably made ofteflon, one role of which is to act as a seal between the wave guide andthe outer environment. FIG. 1 also shows a fluid-filled space 4, betweenthe horn 2 and the transition piece 3, in a circular trench occurringbetween these components. Since the fluid-filled space 4, which in mostcases is filled with condensed water, projects a small distance out ontothe envelope surface of the conical part of the transition piece, amicrowave transmitted through the transition piece interprets the fluidas a restriction, or even a “cut-off”, caused by the fluid reducing theavailable wave guide diameter of the transition piece 3.

A design based on the invention is presented in FIG. 2. It can be seenthat the cylindrical part of the transition piece 3 is constricted sothat it penetrates the horn a distance which is predetermined to beequal to the depth of the circular trench between the horn 2 and thetransition piece 3, which is normally filled with condensed fluid,usually water. By adopting this design the conical part 3 a of thetransition piece 3 will have its base, i.e. its broadest part, atapproximately the level where the fluid-filled space 4 has its externallimits. Thus, the fluid in the space 4 will not expose a wavetransmitted through the transitionpiece to restriction, since the waveguide diameter of the transition piece is not significantly affected byfluid in the space 4.

Moreover, the design according to the invention is self-regulating,since additionally condensed fluid drawn to the space 4 will run off,since capillary action and surface tension will not be capable ofretaining a larger volume of fluid than that contained in the space 4.Furthermore, its operation is not disrupted by the absence of fluid inthe space 4, since this does not influence the wave guide diameter inany significant way.

In the above example both the wave guide and the transition piece areassumed to be circular in cross-section. There is, however, nothingpreventing other cross-sections from being used, for instance, arectangular wave guide cross-section, which in turn requires that theconical part 3 a of the transition piece is pyramidal.

What is claimed is:
 1. A horn antenna for transmitting microwaves,comprising: a conical horn comprising a small opening and a largeopening; and a transition piece comprising a tapering portion and anon-tapering portion, the transition piece being arranged in the conicalhorn between a waveguide and an environment that the horn antenna isarranged in, a transition between the tapering portion and thenon-tapering portion being arranged between the small opening and thelarge opening of the conical horn, and the non-tapering portion engagingthe small opening of the conical horn.
 2. The horn antenna according toclaim 1, wherein the transition between the tapering portion and thenon-tapering portion of the transition piece portion lies outside amaximum level of liquid that in a space defined by the transition pieceand the conical horn can be retained by capillary action and surfacetension.
 3. The horn antenna according to claim 1, wherein thetransition between the tapering portion and the non-tapering portion ofthe transition piece portion lies at a maximum level of liquid that in aspace defined by the transition piece and the conical horn can beretained by capillary action and surface tension.
 4. The horn antennaaccording to claim 1, wherein the transition piece is radiallysymmetrical.
 5. The horn antenna according to claim 1, wherein thetapering portion of the transition piece is conical and the taperingportion of the transition piece is cylindrical.
 6. The horn antennaaccording to claim 1, wherein the tapering portion of the transitionpiece is pyramidal and the tapering portion of the transition piece isrectangular.
 7. A horn antenna for transmitting microwaves, comprising:a conical horn; and a transition piece comprising a tapering portion anda non-tapering portion, the transition piece being arranged in theconical horn such that the tapering portion lies outside a maximum levelof liquid that in a space defined by the transition piece and theconical horn can be retained by capillary action and surface tension. 8.The horn antenna according to claim 7, wherein the transition piece isradially symmetrical.
 9. The horn antenna according to claim 7, whereinthe tapering portion of the transition piece is conical and the taperingportion of the transition piece is cylindrical.
 10. The horn antennaaccording to claim 7, wherein the tapering portion of the transitionpiece is pyramidal and the tapering portion of the transition piece isrectangular.
 11. A horn antenna for transmitting microwaves, comprising:a conical horn; and a transition piece comprising a tapering portion anda non-tapering portion, the transition piece being arranged in theconical horn such that the tapering portion lies at a maximum level ofliquid that in a space defined by the transition piece and the conicalhorn can be retained by capillary action and surface tension.
 12. Thehorn antenna according to claim 11, wherein the transition piece isradially symmetrical.
 13. The horn antenna according to claim 11,wherein the tapering portion of the transition piece is conical and thetapering portion of the transition piece is cylindrical.
 14. The hornantenna according to claim 11, wherein the tapering portion of thetransition piece is pyramidal and the tapering portion of the transitionpiece is rectangular.